Australia's chief veterinary officer Dr Andy Carroll responded to an Australian media report suggesting a horse had tested positive for equine influenza in the quarantine station, saying it was inaccurate.
Carroll said one horse had shown an increase in antibodies for equine influenza since it arrived from overseas, but this did not represent a positive result for the presence of the virus.
Horses that recently arrived at the station from Britain and the United States had developed signs of a respiratory infection, but testing to date indicated it was not EI.
"These horses have tested positive to equine herpes virus 4 which causes a mild respiratory disease, similar to the common cold, in horses," Carroll said.
"Equine herpes virus 4 is already present in Australia and most other countries, and is not a disease of quarantine concern."
With horses showing signs of respiratory disease, additional testing was carried out.
"Routine swabs were taken from all 23 horses on arrival in Australia on June 27 and 28, and again on 3 July. They were all negative to EI.
"Additional, swabs taken from horses that developed high temperatures - and others associated with them - have also tested negative to EI.
"This testing established that equine herpes virus 4 was circulating among these horses," he said.
"As part of this process, suspect horses were also tested for the presence of EI antibodies and one horse has shown an increase in antibodies since it arrived from overseas. This does not represent a positive result for the presence of EI virus.
"Further blood samples and the fifth and final routine swabs for EI virus have been taken from all 23 horses and these have been sent to the laboratory for testing.
"The horses will be held in quarantine until all test results confirm that all horses at the station are EI free."